Because of the linkage of the vitamin nicotinamide to the
ring of the sugar ribose, NAD+ and its relative NADP+ (which carries an extra phospho group in its structure;
Fig. 8) can be reduced by transfer of a hydrogen atom
from an alcohol or other suitable substrate to the 4 position
of the ring. As illustrated in Fig. 8, the transfer is that
of a hydrogen atom plus an electron (a hydride ion H−).
NAD+ plays this role in many biological dehydrogenation
reactions which convert various alcohols into the corresponding
carbonyl compounds—aldehydes or ketones.
At the same time, many carbonyl compounds are reduced
to alcohols. Sometimes the oxidation and reduction processes
are linked. A well-known example is the oxidation
of glyceraldehyde 3-phosphate during the breakdown of
glucose,aprocess that occurs inbacteria, yeast, andthehuman
body. In all cases NAD+ is reduced to NADH + H+.
The latter is reoxidized to NAD+ in the human body, but
in lactic acid bacteria the NADH is used (always together
with an H+ ion) to reduce pyruvic acid to lactic acid. This
provides a balanced fermentation process that requires no
oxygen. Under conditions of extreme exertion, e.g., in a
100-meter race, the lactic acid fermentation fuels human
muscles. In yeast, a similar fermentation reduces acetaldehyde
to ethanol, indirectly providing energy for the cell.

Why are there two similar coenzymes NAD and
NADP? A generalization that holds in many instances is
that NAD+ initiates dehydrogenation (oxidation) while
NADPH acts as a biological reductant. This permits oxidative
pathways utilizing NAD+ to occur at the same time
as reductive processes that utilize NADPH. Cells of aerobic
organisms often keep the concentration ratio of the
reactants [NAD+]/[NADH] high at the same time that the
ratio [NADPH] / [NADP+] is also high. Nicotinamide is
a very stable compound, but the coenzyme forms are surprisingly
easily destroyed. The reduced forms NADH and
NADPH are extremely unstable below pH 7, undergoing
ring opening reactions. NAD+ and NADP+ are unstable
at high pH, hydroxide ions adding to double bonds in
the nicotinamide ring with subsequent destruction of the
coenzymes. It is not surprising that our bodies need a daily
supply of this vitamin.

Like NAD+, FAD and the simpler riboflavin monophosphate
(FMN) often serve as an acceptor of a hydride (H−)
ion. However, FAD is a more powerful oxidant than is
NAD+. This fact is indicated in a quantitative way by
the standard reduction potential, which biochemists tabulate
for pH 7. At this pH the standard hydrogen electrode
potential E0´ (for the couple H+/H2) is −0.414 V while
that for the powerful oxidant O2 (O2/H2O) is +0.815 at
25°C. For the NAD+/NADH couple E0´ is −0.32 V and
for FAD/FADH2 it is −0.21 V. However, since FAD and
FADH2 are often tightly bound as flavoproteins, the value
of E0´ for flavoproteins varies over a broad range from
−0.49 to +0.19 V. The value depends upon the relative
strength of binding of the oxidized and reduced forms of
FAD to the specific catalytic proteins. In the β oxidation
of fatty acids (Fig. 12), the powerful oxidizing properties
of FAD make it possible to remove a C3 hydrogen atom
as H− either after or concurrently with the removal of a
proton from C2. The latter requires participation of a basic
group from the protein as well as activation by the CoA
thioester group (step b in Fig. 12). The thioester group
also facilitates addition of an HO− ion at the C3 position
in step c to form an alcohol. The latter is dehydrogenated
by NAD+ in step d.

Another important aspect of FAD chemistry is the ability
to accept a single hydrogen atom (or a single electron
together with a proton) to form a free radical, which we
may designate FADH•; the dot indicates the reactive unpaired
electron. This ability allows FAD or FMN to accept
a hydride ion, undergoing a two-electron reduction, then
pass the electrons one at a time to an electron-accepting
metal center in an electron transport chain such as that
found in membranes of the mitochondria. It is at the
ends of these electron transport chains that oxygen (O2),
brought into the human body through the lungs, combines
with four electrons and four protons to form two water
molecules. At the other end of the chain −OH groups in
a variety of metabolic intermediates are dehydrogenated
to carbonyl groups by molecules of NAD+. The resulting
NADH transfers its hydrogen (plus a free H+) to FMN
within the mitochondrial chain. These reactions, which
pass electrons through the electron transport chain, account
for most of the oxygen utilized in respiration.

The ability to accept single electrons also allows FAD or
FMN attached to some enzyme proteins to react directly
with O2, reducing the O2 to hydrogen peroxide, H2O2.
The latter has useful functions within cells but may also
cause damage. Molecular oxygen (O2) combined chemically
with the reduced riboflavin is also used by hydroxylases
of bacteria and plants to introduce −OH groups into
a variety of compounds. A peroxide form of FMN, when
bound to the correct protein of luminous bacteria, emits
visible light.

Living cells contain many other hydrogen and electron
carriers. Among them are lipoic acid (Fig. 11), quinones
such as vitamin K, ubiquinone and plastoquinone (Fig. 3),
and metal centers containing iron, copper, nickel, manganese,
and cobalt.